1. Field of the Invention
The present invention relates to a linear motor. More particularly, the invention pertains to a linear motor used in a table feed mechanism of a machine tool.
2. Description of the Background Art
FIG. 20 is a cross-sectional diagram showing the construction of a conventional linear motor disclosed in Japanese Laid-open Patent Publication No. 2000-217334.
Referring to FIG. 20, a stator 1 includes a plurality of permanent magnets 3a, 3b arranged in a line at regular intervals on a stator yoke 2 in alternately reversed directions to produce alternating polarities. A moving part 4 moves along the stator 1 as if sliding over the stator 1 with a specific distance (gap) therefrom.
The moving part 4 includes a moving yoke 5, connecting parts 7 each having a trapezoidal cross section which are held at specific intervals on one side of the moving yoke 5 facing the stator 1 by bolts 6 fitted in the moving yoke 5, a plurality of magnetic teeth (poles) 8 generally T-shaped in cross section and joined to the individual connecting parts 7 which are fitted into dovetail grooves 8a formed in a central part of each tooth end, each magnetic tooth 8 having a recess 8b and a protrusion 8c formed on opposite sides, and a plurality of magnetic teeth (poles) 9 generally I-shaped in cross section and fitted between the successive magnetic teeth 8 as a recess 9a and a protrusion 9b formed on opposite sides of each magnetic tooth 9 fit over and into the protrusion 8c and the recess 8b of the adjoining magnetic teeth 8, respectively. Also included in the moving part 4 are coils 10 individually wound around the magnetic teeth 8, 9 and a resin molding 11 surrounding the magnetic teeth 8, 9 and the coils 10 to join them together into a single structure.
In the conventional linear motor thus constructed, the moving part 4 is assembled by first winding the coils 10 around the individual magnetic teeth 8, 9. Next, the dovetail grooves 8a formed in the individual magnetic teeth 8 are fitted over the respective connecting parts 7 held by the bolts 6 fitted in the moving yoke 5 by sliding the magnetic teeth 8 in a direction perpendicular to the plane of the paper (FIG. 20) and, when the magnetic teeth 8 have been set into position, they are fixed to the moving yoke 5 by tightening the bolts 6. Then, the individual magnetic teeth 9 are slid between the successive magnetic teeth 8 with the recess 9a and the protrusion 9b formed on each magnetic tooth 9 meshed with the protrusion 8c and the recess 8b of the adjoining magnetic teeth 8, respectively. Finally, the alternately arranged magnetic teeth 8, 9 and the coils 10 are joined together into a single structure by the resin molding 11.
Since the conventional linear motor is assembled by inserting the magnetic teeth 9 between the successive magnetic teeth 8 as stated above, the coils 10 wound around the magnetic teeth 9 slide over the coils 10 wound around the magnetic teeth 8 with friction. This assembly process could cause damages to the coils 10, such as an insulation failure or a wire breakage, resulting in a reduction in reliability.
Furthermore, the conventional linear motor is associated with a poor labor efficiency problem. This is because its assembly involves rather complicated procedures including fitting and sliding the dovetail grooves 8a formed in the individual magnetic teeth 8 over the respective connecting parts 7, tightening the bolts 6 to fix the magnetic teeth 8 to the moving yoke 5, mating the recess 9a and the protrusion 9b formed on each magnetic tooth 9 with the protrusion 8c and the recess 8b of the adjoining magnetic teeth 8 and sliding them to fit the magnetic teeth 9 between the successive magnetic teeth 8.
Generally, magnetic teeth are manufactured by stacking press-cut electromagnetic steel sheets. Accordingly, the stacking thickness of the electromagnetic steel sheets should be increased if it is necessary to increase the width of the individual magnetic teeth due to an increase in motor capacity. An increase in the stacking thickness tends to cause an inclination of the stacked electromagnetic steel sheets due to stacking errors as well as a deterioration in assembling efficiency. In addition, it is necessary to increase the thickness of a lower press die if the stacking thickness increases. This would lead to an increase in cost for making the die and an eventual rise in manufacturing cost of the magnetic teeth.
Even when the structure of magnetic teeth does not adopt the aforementioned steel sheet stacking design, it is still necessary to vary the width of the individual magnetic teeth with changes in motor capacity, and this makes it difficult to attain desirable levels of efficiency with respect to the control of production and inventory of various components.